. . . And, at the End of Days, Wisdom shall be lost as Justice falls upon the world of men.

Valor shall turn to Wrath -

and all Hope will be swallowed by Despair.

Death, at last, shall spread its wings over all -

as Fate lies shattered forever.

Considering the End of Days prophecy is slowly becoming true, with Tyrael casting himself from heaven(Justice falling amongst the world men) Malthaelial becoming the arch angel of death...I think you may be on to something here xD

But alas... this is all my own speculation... then again, with the expansion most likely being "Reaper of Souls" and my theory that ArchAngel Malthael becoming the Angel of Death...I might be right ;)

Considering the End of Days prophecy is slowly becoming true, with Tyrael casting himself from heaven(Justice falling amongst the world men) Malthaelial becoming the arch angel of death...I think you may be on to something here xD

He kinda is trapped in the Soulstone. HIs "essence". If they destroy the Soulstone at the Hellforge, he would get back in time. That's why they hid the Soulstone in the first place and that's why in the animation called "Wrath" they wanted to imprison him.

Short answer: no. Diablo, as a demon, cannot die. Regardless of whether everything after the phantasm realm is all an elaborate hoax or real, his "soul" would continue to exist in the black soulstone. If it were not there, for whatever reason, his soul would go to whatever black pit demon souls go to regenerate for a while.

Either way, he would be back. I believe the point of the soulstones were that they could basically pause the regeneration effects forever, so they would stay as close to dead as possible for them.